1. Field of the Invention
The present invention relates to a thermistor device, which comprises a thermistor element body and an ohmic electrode provided on its major surface.
2. Description of the Background Art
In a positive characteristic (PTC) thermistor and a negative characteristic (NTC) thermistor, a structure is known, comprising a thermistor element body and ohmic electrodes, mainly composed of silver or the like, which are provided on both major surfaces thereof. FIGS. 10 and 11 are respectively a perspective view and a sectional view showing a conventional thermistor device having such a structure. Referring to FIGS. 10 and 11, a PTC thermistor comprises a thermistor element body 1 which is made of a ceramic material such as barium titanate, and ohmic electrodes 2 and 3, mainly composed of silver or the like, which are provided on both major surfaces thereof. In such a thermistor device, the ohmic electrodes 2 and 3 are directly exposed to the outside air. When potential difference is developed across the ohmic electrodes 2 and 3, therefore, electrode components forming the ohmic electrodes 2 and 3 may deposit on an outer peripheral surface 1c of the thermistor element body 1 and move to approach each other, to cause shorting across the electrodes 2 and 3. Such a phenomenon is generally called migration, which is very likely to be prompted particularly in a hot and humid atmosphere.
FIGS. 12 and 13 are respectively a perspective view and a sectional view showing the structure of another conventional thermistor device which is known in the art. Referring to FIGS. 12 and 13, this conventional thermistor device comprises a thermistor element body 1 and ohmic electrodes 2 and 3, having smaller areas than the major surfaces of the thermistor element body 1. The ohmic electrodes are provided on the major surfaces. Gap portions 1a and 1b are defined between outer peripheral edges of the ohmic electrodes 2 and 3 and those of the thermistor element body 1. In the thermistor device having such a structure, it may be possible to prevent shorting caused by the aforementioned migration by defining sufficiently wide gap portions 1a and 1b. In this case, however, chipping results from a difference in stress between heating and non-heating portions of the thermistor element body 1 caused by thermal expansion, or stress caused by electric field concentration.
In order to prevent such shorting caused by migration, Japanese Utility Model Laying-Open No. 62-76504 (1987) proposes a structure of a thermistor device comprising a thermistor element body and resin coating layers provided on its outer peripheral surface. FIG. 14 is a sectional view showing the thermistor device having such a structure. As shown in FIG. 14, a resin coating layer 4 is provided on an outer peripheral surface of a thermistor element body 1.
However, this structure is not suitable for practical application since the application of such a resin coating layer provided on the outer peripheral surface of a thermistor element body requires a complicated step, to increase the cost.